'Spoilers for Rule of Two.'
While it isn't a duel strictly speaking (a duel is a fight between two individuals), my favourite would have to be Bane and Zannah vs Farfalla, Johun, Raskta, Worrer and Sarro.
That was probably the most thrilling lightsabre battle I've ever read. My favourite part would have to be Zannah vs Sarro.
Sarro is a giant who's bigger and stronger (physically) than Bane, is armed with a massive sabrestaff that gives him great reach, is an elite fighter (described as having "elevated the art of combat to it's purest and highest form"😉 trained by the Jedi Weapons Master and best duellist of her Era, AND his powers are being boosted by Worrer's Battle Meditation. Despite all that Zannah still holds her own against him and when he's distracted for a second, she uses sorcery to beat him.
Awesome!